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Help : isolated system-conservation of mechnaical energy

  1. Jan 15, 2004 #1
    can canyone here help me with this physics problem:
    Air moving at 11.0 m/s in a steady winds encounters a windmill of diameter 2.3m and having an efficiency of 27.5 %. The energy generated by the windmill is used to pump water from a well 32.5m deep into a tank 2.30m above the ground. At what rate in liters per minute can water be pumped into the tank?

    This is what I have so far:
    Ekin = 1/2mv^2
    Density for air is: 1.29 kg/m^3
    The time for this energy to form is 1 sec.
    m = ( a * v * A ) ..... where A is air density and v is velocity, a in this case is area of the windmill, which is pi(diameter/2)^2

    Ekin = 1/2 (a * v * A)

    So :
    Ekin * 25% = energy generated by the windmill

    After that, I'm lost. But I think it has to do something with potential energy.
  2. jcsd
  3. Jan 15, 2004 #2


    User Avatar
    Science Advisor

    That should be v^3
    The energy needed to raise mass m a height h is mgh where g is acceleration of gravity (9.8 m/sec/sec). This should give you enough info to do the problem.
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